The three elements of electromagnetic interference are interference source, transmission method and disturbed equipment. Therefore, the suppression of electromagnetic interference should also start from these three aspects. First, suppress the interference source and directly eliminate the interference cause; secondly, eliminate the coupling and radiation between the interference source and the disturbed device, and cut off the propagation path of electromagnetic interference; thirdly, the anti-interference ability of the disturbed device should be improved, and its impact on sensitivity to noise.
Using power factor correction (PFC) technology and soft switching power conversion technology can greatly reduce the noise amplitude.
Firstly, measures on the circuit
The main reason for the electromagnetic interference of switching power supply is the sharp change of voltage and current, so the rate of change of voltage and current in the circuit must be reduced as much as possible (DU/DT, di/DT). Absorption circuits are also a good way to suppress electromagnetic interference. The basic principle of the absorption circuit is to provide a bypass for the switch when the switch is turned off to absorb the energy accumulated in the parasitic distribution parameters, thereby suppressing the occurrence of interference. Commonly used absorption circuits are RC, RCD, LC passive absorption network and active absorption network.
Filtering is a good way to suppress conducted interference. Reasonable design and selection of filters, and correct installation of filters are an important part of anti-interference technology.
The specific measures are as follows:
- A common mode noise filter is installed at the output of the power supply. A ferrite magnetic ring is set on the output line to form a conjugate choke coil, and then a high-frequency capacitor is added to suppress part of the common mode noise. Increasing the inductance of the output filter inductor and the capacitance of the filter capacitor can suppress differential mode noise, and the effect of connecting multiple capacitors in parallel is better.
- The output rectifier diode adopts multiple diodes in parallel to share the load current, select the rectifier diode with soft reverse recovery current, appropriately reduce the turn-on speed of the switch tube, reduce the leakage inductance of the high-frequency transformer, and ensure that it is not saturated.
- Install an RC absorption network between the primary side and the secondary side of the high-frequency transformer, the CE pole of the switch tube and the output rectifier diode. Suppresses voltage spikes and current surges. A saturable amorphous magnetic ring is connected in series in the branch of the output rectifier diode to suppress the reverse surge current of the diode.
- When typesetting and printing, the area of the high-frequency loop should be minimized and the high-frequency signal line should be shortened. Attention should also be paid to the wiring of the whole machine: the input AC power cord and the output DC power cord should not be placed together, let alone tied together, and should be kept away from noise sources as far as possible.
The output DC power line should be twisted pair, at least close to each other. The input and output power lines of the power supply should be kept as far away as possible from the signal lines in the control and drive circuits.
- The distributed capacitance CI between the collector of the switch and the heat sink should be minimized. An insulating pad with low dielectric constant can be selected, and the thickness of the pad is appropriately thickened. If necessary, insert a thin copper plate between insulating pads for electrostatic shielding.
- Grounding, one purpose of power grounding is for safety, and the other is to consider electromagnetic compatibility. A good grounding system can go a long way in reducing electromagnetic interference.
Secondly, structural measures: shielding
Shielding is one of the important means to solve the electromagnetic compatibility problem, and its purpose is to cut off the propagation path of electromagnetic waves. Most EMC problems can be solved by electromagnetic shielding. The biggest advantage of using electromagnetic shielding to solve the problem of electromagnetic interference is that it will not affect the normal operation of the circuit. Shielding is divided into electrical shielding, magnetic shielding and electromagnetic shielding.
How to use noise suppression sheet
The most direct way to solve the noise suppression sheet is to paste the noise suppression sheet at the noise source, which can suppress the spread of noise, and paste it on the cable and circuit, which can effectively suppress the conducted noise.
Although the noise suppression sheet is extremely light and thin, it has a high EMI noise suppression effect. At the same time, it is flexible and practical, and can be used in corner positions.
The noise suppression sheet is very suitable for the increasingly thin and miniaturized electronic products. In addition, all the materials constituting this product are non-magnetic, and there is no need to limit the use site.
The test of the noise suppression sheet is mainly adapted to the standard of IEC62333-2. The test of the noise suppression sheet in the standard is mainly through the decoupling ratio of the same side, the decoupling ratio of the opposite side, the transmission attenuation ratio, the radiation suppression ratio and the line decoupling.
The noise suppression sheet is used to reduce noise in digital images. First, at least one luminance threshold value is provided; then, at least one luminance feature value is determined according to the luminance value of a target pixel and the luminance values of adjacent pixels of the target pixel; then, the noise suppression sheet compares the luminance feature value with the luminance The threshold value is used to determine whether the target pixel is a noise point; finally, when the target pixel is a noise point, the brightness value, the first chroma value and the second chroma value of the target pixel are adjusted. By inventing noise suppression The method can not only find out the noise in a digital image, but also reduce the damage and interference caused by the noise itself to the image, thereby improving the picture quality of the image.
For switching power supplies, the shielding effectiveness is mainly improved by shielding rectifier tubes, switching tubes and various switching tubes. These are the two suppression directions of switching power supply noise. If you want to know more about noise suppression sheet or emi suppressor sheet, please contact Penghui, we will let professionals answer for you.
